How to add hollows on the surface of mesh and don't make it too complicated for future modelling
Composition.  One might think a sphere/cylinder needs more vertices than a cube. If you place a sphere next to a cube and both are children of an empty then each can have different numbers of vertices mininal to their shape. They might not share any vertices at all.  The location closeness can make them seem as single connected mesh even though they are disconnected.
Consider searching Blender Parent Empty at some well known video website.
See videos on Blender armatures if rotation is necessary.
Single Mesh Example in edit mode.  The sphere vertices have not effect on the cube vertices.





The above can be a single mesh.  Yet the composed meshed share no vertices.
I see textures in your image above.  For low levels of detail you can replace vertices with textures.  All the metal texture and decal paint appearing in your image are probably usefully and sanely done with Blender textures.  You can use a texture to imitate a protusion or indentation especially if that area will not be seen close up.
Another search here will probably locate more questions on the same topic.